In the electronic industry, to impart solder receptivity to copper or copper alloy portions on electronic parts and circuits, it has been a common practice to form a tin, lead or tin-lead alloy coating on the copper or copper alloy portions by electroplating techniques. As electronic devices are reduced in size, parts and circuits are also miniaturized or complicated. Some finely defined portions are difficult to plate by electroplating techniques. Attention has been paid to electroless tin, lead or tin-lead plating techniques capable of plating on such finely defined portions. For example, Japanese Patent Application Kokai No. 184279/1989 proposes the use of an electroless tin-lead alloy plating bath predominantly containing a specific organic sulfonic acid, tin and lead salts thereof, sodium hypophosphite (reducing agent), and thiourea (complexing agent). However, most prior art electroless tin, lead or tin-lead alloy plating systems are designed as batchwise disposable systems in that metal replenishment is not contemplated, and the plating bath is discarded simply when the metal concentration falls below the deposition limit. Such systems are employed mainly when it is desired to form thin plating films. Few proposals have been made for the continuous operation of an electroless plating bath for the purpose of establishing thick plating films. No study has been made of the controlled replenishment of an electroless plating bath able to maintain the constant rate of deposition which is essential for continuous operation.
In ordinary electroless plating processes, the plating baths are managed by analyzing the bath for metal components, and replenishing necessary metal components to the bath in proportion to their consumption. We found that it is difficult to simply and accurately analyze tin and lead components in an electroless tin, lead or tin-lead alloy plating bath for the plating of copper-containing articles, because with the progress of plating, copper is dissolved out of the articles and gradually accumulated in the bath. Therefore, the conventional electroless plating bath management is hardly applicable to the electroless tin, lead or tin-lead alloy plating bath.
Therefore, there is a need for an electroless tin, lead or tin-lead alloy plating process capable of controlling the content of tin, or lead in an electroless tin lead or tin-lead alloy plating bath in a simple, but consistent manner, thereby facilitating continuous plating of thick films.
The current mount technology of electronic parts like integrated circuit packages is undergoing a transition from the vertical mount technology (VMT) of DIP type packages to the surface mount technology (SMT) of flat packages. Accordingly, printed circuit boards are desired to present a smooth surface in contact with IC packages and other parts to be mounted, that is, to have a highly uniform tin, lead or tin-lead alloy plating film on copper or copper alloy circuit conductors.
The conventional electroless tin, lead or tin-lead alloy plating baths, however, are less stable at the initation of operations, and tend to deposit films of coarse grains, which are poor in uniformity of film thickness and alloy composition. That is, the conventional electroless plating baths are likely to produce tin, lead or tin-lead alloy films which are defective in outer appearance or reflow at the initial stage and resultin find difficulty in the plating of small-size parts such as SMT-oriented fine pitch printed circuit boards and chip parts.
Therefore, there is a need for an electroless tin, lead or tin-lead alloy plating process capable of producing a uniform plating film throughout the service life of a plating bath from the initial formulation to the end of long-term operation and capable of accommodating SMT-oriented fine pitch printed circuit boards and chip parts.